The invention relates to an optical system comprising a cavity for spectrophotometric measurements and the like, using optical fibers for the conveyance of light radiations from an emitting source to the cavity containing the sample on which the measurments must be carried out, and from this to devices for the detection of the modulated radiation.
Various types of optical fiber sensors are based on spectrophotometric measurements carried out on liquids or gases. In these sensors, a suitably modulated and filtered light radiation is conveyed, via an optical fiber, into a cavity containing the medium, generally a liquid or a gas, to be tested for measurement. The measurements are based on the detection of absorbance variations in the medium under test, according to the variation of a specific physical or chemical quantity, such as for example, the temperature, the concentration of a specific component of a mixture, or similar.
The absorbance measurement with optical fibers is usually carried out by utilizing two fibers, one facing the other, one being used for illuminating the sample under test, i.e. for conveying the light radiation from the source to the measuring volume or cavity, and the other being used for picking up the light modulated by the substance under test interposed between the two fibers. In some cases the system comprises only one fiber facing a mirror, or a pair of fibers facing a mirror, the substance under test being interposed between said fibers and the mirror. In these cases, the light radiation passes through the substance under test, is reflected by the mirror, passes through the substance under test again, and is then picked by the same optical fiber which has emitted it or by a fiber adjacent to the latter. In any case, owing to the divergence of the radiation beam emitted by the fiber, the fraction of light which is picked up is very small. Moreover, even small disalignments of the two fibers or of the mirror cause significant reductions of efficiency.
The problems are the more serious the longer the cavity is or the greater volume inside which the measurement has to be performed. In some types of sensors and for particular measurements, in fact, it is necessary that the radiation emitted by the optical fiber goes through a cavity which is some decimeters long, at the opposite end of which--with respect to the end where the radiation input optical fiber is disposed--there must be arranged a flat mirror which is perfectly perpendicular to the axis of the optical fiber at the output section thereof, or a second optical fiber must be provided perfectly aligned with the first one. Even very small disalignments--due to assembly errors, work tolerances, thermal expansions, or the like--between one fiber and the other or between a fiber and the mirror, may cause the complete scattering of the beam.